1. Field of the Invention
The present invention relates to a mobile communication terminal, mobile communication system, base station, and a communication method for use by them, and more particularly to a control technique for controlling the transmission timing of control information to be transferred between a mobile communication terminal and a base station.
2. Description of the Related Art
FIG. 1 shows the configuration of a UMTS (Universal Mobile Telecommunications System), a mobile communication system standardized by the 3GPP (3rd Generation Partnership Project). The radio communication system 1 comprises a radio access network N1 called a UTRAN (Universal Terrestrial Radio Access Network), a circuit switching core network N2, which connects the radio access network N1 and a public switch telephone network N3 to provide radio switched services, and a packet switching core network N4, which connects the radio access network N1 and an Internet protocol (IP) network N5 to provide packet switched services.
The radio access network N1 comprises base stations BSs and radio network controllers 2 (RNCs) for controlling the BSs, and is responsible for transferring user information, such as voice and packets from mobile user terminals (UEs) to core networks N2 and N4 and vice versa and for allocating radio resources necessary for communications between them.
The circuit switching core network N2 comprises a mobile switching center (MSC) 3 and a gateway mobile switching center (GMSC) 4, and is responsible for establishing communication links between terminals by circuit switching.
The packet switching core network N4 comprises: a serving GPRS support node (SGSN) 5, which keeps track of the position of each mobile user terminal UE accessing the packet switched domain and transfers user traffic between a gateway GPRS support node 6 described below, and the radio access network N1; and the gateway GPRS support node (GGSN) 6, which controls the connection between the mobile communication system 1 and the external IP network N5 in accordance with a connection request from the mobile user terminal UE, the packet switching core network N4 thus providing IP connections between mobile user terminals UEs or between mobile user terminals (UEs) and an external IP network.
The radio communication system 1 further includes a home location register (HLR) 7 and an authentication center (AUC) 8, which performs authentication and manages authentication and other confidential information.
When a mobile user terminal UE is connected to a base station BS, the mobile user terminal (UE) intermittently transmits control information to the base station (BS), i.e., downlink quality information (downlink CQI) that indicates the channel quality of a downlink or a downstream channel along which signals are transmitted from the base station (BS) to the mobile user terminal (UE), and an uplink pilot signal to be used for measuring channel quality of an uplink or an upstream channel along which signals are transmitted from the mobile user terminal (UE) to the base station (BS).
The mobile user terminal (UE) receives a downlink pilot signal transmitted via a common pilot channel from the base station (BS), and measures the channel quality of the downlink propagation path. The result of the measurement is transmitted as downlink quality information to the base station (BS). When downstream traffic occurs from the base station (BS) to the mobile user terminal (UE), the base station (BS) thus supplied with downlink quality information can transmit downstream traffic by selecting the transmission format (modulation method, error-correction method, code rate, etc.) that best matches the conditions of the downlink propagation path.
The mobile user terminal (UE) transmits, the upstream pilot signal to the base station (BS) via an upstream dedicated control channel allocated to each individual terminal. The base station (BS) measures the channel quality of the uplink propagation path by measuring the reception condition of the upstream pilot signal. By measuring the channel quality of the uplink propagation path, the base station (BS) can determine the transmission format (modulation method, error-correction method, code rate, etc.) that best matches the conditions of the uplink propagation path.
In the prior art mobile communication system, the transmission timing and carrier frequency of the downlink quality information and the transmission timing and carrier frequency of the uplink pilot signal have been set independent of each other, and this has led to increased power consumption and increased processing complexity of the transmitter circuit in the mobile user terminal (UE), and thus an increased burden on the mobile user terminal (UE). The reason will be described with reference to FIG. 2.
FIG. 2 is a time chart showing a prior art example of how downlink quality information and uplink pilot signal are transmitted. In FIG. 2, the uplink pilot signal (Pilot) is transmitted at times t1 to t2 and t6 to t7, and irrespective of the timing, downlink quality information (CQI) is transmitted at times t2 to t3, t4 to t5, and t8 to t9.
When transmitting the downlink quality information and uplink pilot signal at such times, the mobile user terminal (UE) has to drive its transmitter circuit at each transmission timing.
Specifically, in the 3.9-generation mobile communication architecture (3GPP LTE), standardization of which is currently underway, a standby state called the MAC-Dormant state is provided as a state in which the mobile user terminal (UE) can take on, in order to enable data transmission/reception to be initiated upon occurrence of traffic. In the MAC-Dormant state, data transmission/reception is not performed, but the mobile user terminal (UE) transmits the downlink quality information and uplink pilot signal in an intermittent fashion.
In the MAC-Dormant state, when neither the downlink quality information nor the uplink pilot signal is transmitted, power is not supplied to the amplifier circuit in the output stage of the transmitter circuit, and the amplifier circuit is driven only intermittently, thereby reducing power consumption. FIG. 3 shows a state transition diagram for the mobile user terminal (UE) as defined by the 3.9-generation mobile communication architecture. In FIG. 3, LTE-Active state indicates a communication state, and the MAC-Active state contained therein indicates a state in which communication is proceeding continuously, while the MAC-Dormant state indicates a state in which control information such as described above is transmitted and received in an intermittent fashion. Conversely, an LTE-Idle state indicates a standby state in which no control information is transmitted or received, and the LTE-Detached state indicates a state in which the power of the mobile user terminal (UE) is OFF.
When driving the amplifier circuit in an intermittent fashion, the amplifier circuit must be energized for a predetermined standby time, before the input of a transmit signal, in order to stabilize the output characteristics of the amplifier circuit. During this standby time, the amplifier circuit does not amplify the transmit signal, and consumes power unnecessarily. As a result, if the transmission timing of the downlink quality information is spaced apart from the transmission timing of the uplink pilot signal, the amount of power unnecessarily consumed increases, because standby time occurs before each transmission timing.
In the 3.9-generation mobile communication architecture, bandwidths of 1.25 MHz, 2.5 MHz, 10 MHz, 15 MHz, and 20 MHz can be used in addition to the 5-MHz bandwidth used in the current third generation mobile communication systems. FIG. 2 shows a case where the mobile user terminal (UE) conforming to such 3.9-generation mobile communication architecture, transmits the uplink pilot signal using a 20-MHz bandwidth channel and downlink quality information using a 5-MHz bandwidth channel.
When changing the transmit signal frequency, the mobile user terminal (UE) optimizes the frequency characteristic of the amplifier circuit in accordance with the carrier frequency in order to enhance power efficiency. The frequency characteristic of the amplifier circuit is varied by changing the bias to be applied to the active device used in the amplifier circuit. However, there is a limit to how fast the active device can follow the change in bias, and when the amount of bias is large, the amount of delay with which the frequency characteristic of the active device changes in response to the change in the bias becomes large.
As a result, when the amount of change of the transmit signal frequency is large, the bias to the active device must be controlled by taking into account the amount of delay with which the frequency characteristic of the active device changes, and this increases the processing complexity of the transmitter circuit within the mobile user terminal (UE). The same problem also occurs when the frequency bandwidth of the transmit signal greatly changes as shown in FIG. 2.